Elemental Analysis Of The Fine-Grained Fraction Of Lodgement Till In The Cox Brook Drainage Basin, Central Vermont
Jason Messier (ES ’05)
The geochemistry of the fine-grained fraction of lodgment till is controlled not only by local bedrock chemistry, but also to a large extent by the bedrock for tens of kilometers across which the ice flowed. A study done on till from the Cox Brook valley in Northfield-Moretown, Vermont, indicates that local bedrock exerts only minor control on the fine fraction. Samples were taken from cut banks at six locations in the valley ( Fig. 1), and ICAP analyses were performed on the sub-two millimeter fraction.
figure 1
Ten major elements (Na, Mg, Al, Si, Mn, Fe, P, K, Ca, and Ti) and fourteen minor elements (Sc, V, Cr, Ni, Cu, Sr, Zr, Nb, Rb, Ba, Zn, Ag, Cd, and Co) were measured and compared against similar data for bedrock samples from the valley. Two main bedrock formations underlie the valley, the Stowe Formation at the highest elevations the more dominant Moretown formation. The Stowe characteristically is a quartz sericite chlorite phyllite and schist, while the Moretown is an interbedded quartz and quartz-plagioclase granulite. Both formations strike obliquely to the ice flow for the Wisconsinan glaciation that created the Cox Brook till. At its zenith, this glaciation covered the area with up to two miles of ice, and its recession created a series of glacial lakes in the area, the most influential being Glacial Lake Roxbury ( Fig. 2) which filled the lower portions of the valley.
figure 2
Lake Roxbury is most likely responsible for creating tills with anomalous composition at two of the sites (Sites 4 and 6); they consistently represent the maximum or minimum elemental values and are usually inversely proportional. Even with these compositional outliers, a distinct trend is noticeable in the major elements of being reduced from the bedrock content at the top of the valley to being elevated at the bottom of the valley. Sodium content is particularly high and stays consistently high across the valley, despite being the most soluble of the measured elements ( Fig. 3).
figure 3